Gas turbine engines are used as the primary power source for many types of aircraft. Most turbine engines include rotating components such as a fan, a compressor, and a turbine. The rotating components may be clamped together either by a tieshaft or bolted flange joints to form a rotor group. Two or more bearing assemblies may be employed to support the rotor group. Generally, the bearing assemblies may be surrounded by a support housing, which may be connected to an engine case. As higher power to weight ratio engine designs evolve, gas turbine engine rotor bounce critical, that are typically below ground idle, move closer to ground idle and can result in higher engine vibration near idle.
During operation of the gas turbine engine and high-speed rotation of the rotor group, forces may be transmitted from the rotor group to the support housing. To damp the effects of the transmitted forces and minimize the rotor unbalance load transmission, a squeeze film damper may be included in some engines. A squeeze film damper operates by supplying fluid (usually oil) through dedicated oil delivery passages into a squeeze film cavity formed via a clearance between the support housing and the bearing assemblies.
Although squeeze film dampers are relatively useful in reducing rotor vibration in some cases, they may suffer drawbacks in others. For example, squeeze film dampers may not be as effective in case of an uncentered squeeze film, as the weight of the rotor reduces the effectiveness of the damper. In that situation, the support housing stiffness may allow the rotor group to vibrate with certain modes at some operating speed points, for example, the squeeze film damper may not adequately damp at lower speeds, resulting in a rotor-to-structure unbalance response. As a result of the weight of the rotor, the rotating rotor group may not remain concentric with the squeeze film cavity. Thus, the rotor may sit at the bottom of the damper clearance or may be unable to precess around the clearance between the bearing assembly and the support housing. As a result, the squeeze film damper may become relatively stiff during operation and may not absorb as much vibration as desired, High support stiffness sometimes bring a mode in the operating speed of very close to an operating speed increasing vibration level of the engine.
Hence, it is desirable to have an apparatus that may be used to improve the damping capabilities in a gas turbine engine and provide a support structure stiffness that minimizes a rotor-to-structure vibration transmissibility through all engine operating speeds. It is desirable for the apparatus to be capable of maintaining low vibration characteristics for the gas turbine engine for engine speeds ranging from below ground idle to higher operating speeds. It is also desirable for the apparatus to have a relatively compact design and to be capable of being retrofitted into existing engines.